We investigated the expression and production of the interleukin-1 receptor antagonist (IL-1ra) in three human glioblastoma cell lines (LN443, LN444, LN859). Reverse transcription-polymerase chain reaction (RT-PCR) demonstrated the expression of IL-1ra mRNA transcripts in the three cell lines. These three cell lines also expressed mRNA for IL-1 alpha, IL-1 beta, as well as IL-1 receptor type I and type II, suggesting the presence of an IL-1 autocrine loop in these cell lines. Northern blot analysis demonstrated that the IL-1ra mRNA expression increased with IL-1 beta or tumor necrosis factor (TNF)-alpha but not with GM-CSF stimulation in both LN443 and LN444 cell lines. PMA stimulation increased the mRNA expression in LN444 but not in LN443 cells. Immunocytochemical staining showed IL-1ra immunoreactivity in these three cell lines. ELISA on culture supernatants demonstrated that the IL-1ra was secreted from the cell lines in agreement with the mRNA expression. RT-PCR with isoform-specific primers showed that both intracellular and secreted forms of IL-1ra were expressed by the three cell lines, with a predominance of the intracellular form. In vivo study with RT-PCR and Northern blot analysis demonstrated IL-1ra mRNA in six out of 12 human glioblastoma and two out of five anaplastic astrocytoma tissues, although the expression level was not high in some cases. Immunohistochemistry demonstrated the presence of IL-1ra within the cytoplasm of tumor cells in six out of 10 glioblastomas in vivo. These results suggest a potential role of IL-1ra in regulation of the IL-1 autocrine loop in glioblastomas.